This invention generally relates to a separator for electrochemical cells and processes for assembling cells with a separator of this invention. More particularly, this invention is concerned with a thermoplastic separator for use in an electrochemical cell having an alkaline electrolyte.
Cylindrically shaped batteries are suitable for use by consumers in a wide variety of devices such as flashlights, radios and cameras. Batteries used in these devices typically employ a cylindrical metal container to house two electrodes, a separator, a quantity of electrolyte and a seal assembly. Typical electrode materials include manganese dioxide as the cathode and zinc as the anode. The zinc is commonly employed in particulate form suspended in a gel. An aqueous solution of potassium hydroxide is a well known electrolyte. A separator, conventionally formed from one or more strips of paper, is positioned between the electrodes. The electrolyte is readily absorbed by the separator and gelling medium.
One of the essential components of the battery is the separator which must prevent direct electrical contact between the anode and cathode while providing for ionic conductivity therebetween. If the anode and cathode are allowed to physically contact one another, a chemical reaction takes place and the useful electrochemical capacity of the battery is wasted.
In addition to providing an electrically nonconductive and, at the same time, an ionically permeable barrier between the electrodes, a preferred separator must also satisfy several other criteria. First, because the separator does not contribute to the electrochemical capacity of the battery and the volume of most commercially available batteries is limited by industry standards, the separator should occupy the smallest possible volume in order to maximize the volume within the cell that can be occupied by the anode, cathode or electrolyte. To achieve this objective, the ideal separator will be a single layer of thin material that closely adheres to the surfaces of the anode and/or cathode thereby functioning as the interface between the electrodes. Second, a preferred separator can conform to irregularly shaped electrode surfaces. For example, if the interface between the anode and cathode is an undulating surface having recesses and/or projections, the separator must be able to conform to the surfaces of the electrode without tearing, wrinkling or otherwise distorting in an unacceptable manner. Third, the separator's ionic conductivity must be high enough to support the rate of ionic exchange needed to enable high rate discharge of the battery. Fourth, the separator must be tough enough to withstand the rigors of a commercial battery assembly process without tearing or splitting. Fifth, the separator must be able to withstand unacceptable chemical degradation by any of the cell's other components such as the caustic electrolyte or one of the electrode's electrochemically active materials. Sixth, the separator should be melt processable into a wide variety of three dimensional shapes thereby facilitating the use of cell constructions, such as prismatic cells or pouch cells, that do not have a tubularly shaped anode to cathode interface. Seventh, the separator should be processable in commercially available manufacturing equipment, such as an extruder, to enable low cost manufacturing.
A common separator used in commercially available cylindrical alkaline batteries is a strip of paper that has been wound to form a tube. One end of the tube is left open, while the opposite end is sealed or otherwise closed. Unfortunately, a separator formed from a wound tube of paper does not completely satisfy many of the criteria described above. For example, the tube usually incorporates at least two layers of paper thereby resulting in a separator thickness, prior to soakup of electrolyte, of 0.13 mm or more. Although the wound tube is structurally rigid and thus well suited for cell constructions with a tubularly shaped anode-to-cathode interface, the wound tube cannot conform to cell constructions with an irregularly shaped anode-to-cathode-interface. Furthermore, a strip of conventional separator cannot be readily configured into a variety of three dimensional shapes.
Due to the significant economic and technical advantages that can be realized by developing a separator with many of the desirable features described above, separators that incorporate some of the desired characteristics have been developed and documented in the following patents.
U.S. Pat. No. 4,315,062 is directed to a method of forming a separator for use in an electrochemical cell by dissolving polystyrene resin in a solvent to form a solution, placing the solution directly on the surface of the cathode and then removing the solvent thereby leaving a thin, substantially continuous coating on the exposed surface of the cathode. The required use of a solvent is an undesirable step because of the potential environmental hazards that could be created when handling some of the solvents, such as benzene or toluene, which were disclosed in the subject patent as suitable solvents.
FR 2,463,513 discloses a polymer separator resulting from copolymerization of at least one hydrophilic monomer, at least hydrophobic monomer, as well as at least one divinyl monomer. The divinyl monomers are well known crosslinking agents that would inherently cause crosslinking of the hydrophilic and hydrophobic monomers. A crosslinked polymer is not thermoplastic and therefore the polymer cannot be extruded or otherwise physically altered in a melt processable process to obtain a separator that has a desired three dimensional configuration.
U.S. Pat. No. 4,285,751 describes a battery separator that is a thin sheet formed from a substantially uniform mixture of a thermoplastic rubber and a filler in a volume ratio of from about 1:0.15 to 1:0.6. The separator exhibits good electrical conductivity and a high degree of inhibition to dendrite formation. The fillers may be any ingredient which is substantially chemically inert with respect to the electrolyte to which it is to come into contact.
Despite these inventions, there remains a need for a thermoplastic polymer separator that can satisfy the criteria described above when used as a separator in an electrochemical cell that includes an alkaline electrolyte.